In the present description, by a polymerisation process, it is meant a process for polymerising at least one monomer, and a manufacturing process of a polymer is used in the meaning of the whole process, comprising the polymerisation process and any further treatments, such as degassing, additivation, compounding and/or pelletising.
In a polymerisation process, the polymer is typically recovered from the reactor in the form of a polymer powder, either as such or in slurry, depending on the process. The polymer powder also typically contains a certain amount of unreacted monomer and possibly residues of solvent that need to be removed from the powder before further use. This invention relates to cases wherein the monomer and possible solvent is in the form of gas at some point of the manufacturing process. The removal of these undesired gases in such cases is typically made by degassing the polymer powder by flushing it with a flow of flushing gas. In the case of polyolefin manufacture, the gases to be removed are usually hydrocarbon gases such as ethylene, propylene, hexene, butane, octene, decene etc. In this description, the term residual gas is used to designate the mixture of all the products that need to be removed from the polymer powder, including products that are in the form of liquids as well as gases. The term residual gas is also used interchangeably with the term hydrocarbons when discussing their elimination from the system.
One method for flushing a polymer powder consists in introducing the polymer powder in a purge column and injecting a flow of nitrogen gas to said column from the bottom. The nitrogen enters into contact with the polymer powder that in the case of polyolefins has a residence time of about 0.5 to 10 hours in the purge column. A flow of nitrogen and of residual gas exits the purge column via a gas outlet and is directed to an appropriate treatment device via a gas discharge line.
The inventors have found out that in a process of polymerising ethylene, the amount of undesired hydrocarbon gases in the final polyethylene powder (i.e. before pelletising) is too high for some applications, such as for applications where the final product is in contact with food.